Recently, a photocatalytic thin body using titanium oxide has drawn attraction. “Photocatalyst” is a substance having semiconductive physical properties, excited when light with energy higher than the band gap energy between the conduction electron band and charged electron band is radiated, and thereby producing electron-positive hole pairs.
Titanium dioxide with anatase type crystal structure is photo-excited by radiating light with wavelength of 387 nm or shorter and simultaneously causes decomposition reaction based on redox reaction and reaction for hydrophilization reaction different from the decomposition reaction (activation). So far, as a metal oxide simultaneously causing these two reactions, titanium oxide, tin oxide, and zinc oxide have been known and as a metal oxide causing only decomposition reaction, strontium titanate and ferric oxide and as a metal oxide causing only the hydrophilization reaction, tungsten trioxide have been known, respectively.
Self-washing function, deodorization function, anti-bacterial function and the like can be provided by utilizing the above-mentioned reactions and a variety of members and groups of products coated with such a photocatalyst have been proposed.
As a method for producing such a photocatalyst, various methods such as a binder method, a sol-gel method, and a vacuum evaporation method have been proposed.
The binder method involves steps of dispersing finely granular titanium oxide in a binder having an adhesive property, applying the obtained dispersion to a predetermined substrate and heating and drying the applied dispersion. However, the method has a problem that since the finely granular titanium oxide is buried in the binder, the catalytic function tends to be deteriorated.
The sol-gel method is a method for obtaining a photocatalytic film by applying a liquid-phase agent containing a titanium chelate or a titanium alkoxide containing titanium to a predetermined substrate, drying and then firing the agent at a temperature as high as 500° C. or higher. However, the method has a problem that since it requires the firing step at a temperature as high as 500° C. or higher, materials usable as the substrate are considerably limited in terms of the heat resistance.
Besides these formation methods, formation methods by using a vacuum evaporation method or sputtering method have been proposed.
For example, in JP Patent No. 2,901,550, a photocatalyst having a layered structure of titanium oxide and silicon oxide by a vacuum evaporation method is disclosed.
Also, in Japanese Patent Application Laid-Open No. 2000-126613, a method for depositing silicon oxide by reactive sputtering is disclosed.
Meanwhile, on the basis of evaluation results of various physical properties of titanium oxide films formed under variously different conditions by sputtering, the inventors of the invention have found that a titanium oxide film formed under conditions different from those of conventional methods has a particular structure different from conventional structures. The inventors have found that with respect to such a titanium oxide film, the photocatalytic property is remarkably improved as compared with that of a conventional film.
The invention is accomplished based on such findings and an object of the invention is to provide a photocatalyst having an excellent photocatalytic property and simultaneously having a good dark place retention property and improved productivity, a photocatalyst production method and a photocatalyst production apparatus.